The flash memory which is a typical nonvolatile semiconductor memory device is excellent in portability and shock resistance, and can perform a bulk erase. Thus, demands for flash memory as a file memory for small-sized portable information devices have been rapidly growing. In general, a flash memory is configured by disposing a plurality of memory cells in a matrix form, in which each of the memory cells has a field-effect transistor as one storage unit, and the field-effect transistor includes a silicon substrate having source and drain diffusion layers, a floating gate and a control gate formed on this silicon substrate and mainly made of a polycrystalline silicon film, an interlayer insulating film for separating these gates, and a tunnel insulating film for separating the floating gate and the silicon substrate.
Also, a memory cell including, in addition to the floating gate and the control gate, a third gate (hereinafter referred to as an assist gate) formed of a polycrystalline silicon film and having functions as a device isolation between memory cells and a write gate or an erase gate, and an interlayer insulating film for separating the floating gate and the assist gate has been reported. In such memory cells, a positive voltage is applied to a control gate on the substrate to inject electrons into the floating gate, and information of “0” or “1” is determined from a difference in threshold voltages.
Initially, as the interlayer insulating film for separating the floating gate and the control gate, a silicon oxide film formed by thermally oxidizing a polycrystalline silicon film forming the floating gate has been used. However, this silicon oxide film has a dielectric withstand voltage lower than that of a silicon oxide film formed on the silicon substrate through a thermal oxidation process, and when applied to a flash memory, it becomes one of the factors to deteriorate a charge retention characteristic. For its solution, in 4 megabits and following flash memories, a multi-layer film having a silicon nitride film vertically interposed between silicon oxide films (hereinafter referred to as an ONO film) has been used.
For example, a method of manufacturing a nonvolatile semiconductor memory device is disclosed in Japanese Patent Application Laid-Open Publication No. 2001-15619 (Patent Document 1), in which a silicon oxide film is formed on a floating gate through CVD, a nitride layer is formed by introducing nitrogen near an interface between the floating gate and the silicon oxide film, a silicon nitride film is formed on the silicon oxide film through low pressure CVD, and a control gate is formed on the silicon nitride film.
Also, for the interlayer insulating film for separating the floating gate and the assist gate, various insulating films have been suggested. For example, a read-only memory transistor is disclosed in Japanese Patent Application Laid-Open Publication No. 2-110981 (Patent Document 2), in which the interlayer insulating film is composed of a silicon oxide film formed through thermal oxidation of a polycrystalline silicon film. Moreover, a semiconductor integrated circuit device and its manufacturing method are disclosed in Japanese Patent Application Laid-Open Publication No. 2001-28428 (Patent Document 3), in which the interlayer insulating film is composed of a silicon oxide film to which nitrogen is added. Furthermore, a method of manufacturing a nonvolatile semiconductor memory device is disclosed in Japanese Patent Application Laid-Open Publication No. 5-190862 (Patent Document 4), in which a nitride film, an oxide film, and a nitride film are laminated on a sidewall of a select gate electrode to form a nitride-film sidewall insulating film, and then a floating gate electrode is formed on the nitride-film sidewall insulating film.